1. Field of the Invention
The present invention relates to a thermoelectric conversion device having elements constituted of p-type and n-type thermoelectric materials and capable of enabling temperature-difference power generation (thermal power generation) based on the Seebeck effect or electronic cooling and heating based on the Peltier effect and a method of manufacturing.
2. Description of the Related Art
The construction of a conventional thermoelectric conversion device for converting heat into electricity or converting electricity into heat and a method for manufacturing the thermoelectric conversion device will be described with reference to FIGS. 14 to 16. The conventional thermoelectric conversion device is constructed in such a manner that, as shown in FIG. 14, n-type elements 505 and p-type elements 605 are interposed between a lower substrate 601 having a lower electrode wiring 604 provided on its surface and an upper substrate 501 having an upper electrode wiring 504 provided on its surface. Also, as shown in FIG. 16, p-type elements 605 and n-type elements 505 are electrically connected in series to each other to form pn junctions, and are connected to an external electrical circuit by external connection electrodes 608. Thermoelectric materials ordinarily used at room temperature not exceeding a temperature of about 200xc2x0 C. are Bixe2x80x94Te materials. The material of the p-type elements is a chemical compound semiconductor mainly composed of Bi, Te and Sb. The material of the n-type elements is a chemical compound semiconductor mainly composed of Bi, Te and Se.
When, in the above-described thermoelectric conversion device, a direct current is caused to flow through the external connection electrode 608 provided at both end portions of the thermoelectric conversion device, heat absorption or heat development occurs at each of the interface at which the upper electrode wiring 504 contacts the elements 505 and 506 and the interface between at which the lower electrode wiring 604 contacts the elements 505 and 506, thus creating a temperature difference between the two faces of the thermoelectric conversion device. Conversely, when there is a temperature difference between the upper electrode wiring 504 and the lower electrode wiring 604, it is possible to extract power from the external connection electrodes 608.
A method for manufacturing such a thermoelectric conversion device will next be described. FIG. 15 shows an element joining method in a conventional thermoelectric conversion device manufacturing method in a case where elements formed from thermoelectric materials are monocrystals, or where a sintering process is used. First, on a thermoelectric material processed to have the shape of a plate or rod, a layer of Ni or the like is formed by plating. This layer is formed on the flat surfaces of the thermoelectric member which are to be joined to the substrates. The thermoelectric member is cut into rectangular blocks, and electrode joint layers for soldering are provided on opposite end surfaces. In this manner, p-type elements 605 having electrode joint layers 606 and 607 and n-type elements 505 having electrode joint layers 506 and 507 are made. Then, the completed p-type elements 605 and n-type elements 505 are respectively set in predetermined places on the electrode wirings with a jig or the like, and the elements and the electrode wirings are joined to each other by the electrode joint layers, thus manufacturing a thermoelectric conversion device. FIG. 16 shows a see-through view of the thermoelectric conversion device manufactured by this process.
Ordinary conventional thermoelectric material forming methods are represented by the above-mentioned method of directly cutting a monocrystal into elements, and the method of pulverizing a monocrystal into a powder, sintering the powder and cutting the sintered material into elements. Sol-gel methods, electroplating methods, flash evaporation methods are presently being studied.
A process based on an electroplating method disclosed in Japanese Patent Laid-Open Publication No. 22533/1998 will be outlined. First, a mask pattern is formed on a plating electrode made of Ti or the like on a plating substrate. This plating substrate is then placed in an acid plating solution together with an opposed electrode and a current is caused to flow therethrough.
When the growth of a plating layer to a certain thickness is attained, the substrate with the plating layer is taken out of the liquid plate and the plating layer is transferred onto an insulating substrate. As a transfer method, a method of physically shaving the plating substrate may be used. In this process, however, a method of stripping from the plating substrate by using the adhesion of an adhesive on the insulating material is used. The step of forming an insulating layer on the stripped off plating layer and transferring another plating layer onto the insulating layer is repeated several ten times to make a laminated block. Internal electrodes for pn junction are directly formed on end surfaces of the block by vacuum deposition or the like.
In the thermoelectric conversion device using monocrystals or sintered members as its elements, however, shorting by contact between the elements or failure of contact of the elements with the electrodes can occur easily due to a move of the elements on solder at the time of joining when the both sides of the elements are soldered to fix the elements. Also, since the elements having the shape of a rectangular block are made by cutting the thermoelectric material in the form of a plate, corner portions of the elements can crack or chip easily and a stress due to heat or an external force is concentrated in corner portions of the elements to increase the possibility of breakage. Therefore, there is a problem of a reduction in the yield or instability of the yield in mass production of the thermoelectric conversion device.
In the thermoelectric conversion device manufacturing method using a monocrystal or a sintered member to form elements, the elements formed from a thermoelectric material is a rectangular block one side of which (one in the direction of thickness and one of the bottom sides) has a length of, at the minimum, several hundred micrometers, depending upon restrictions in terms of machining and ease of handling. To obtain a high electromotive force by using such elements with respect to a small temperature difference, it is necessary to connect several thousand elements in series. However, it is very difficult to manufacture a thermoelectric conversion device in which several thousand worked elements are arrayed one by one on a substrate and connected in series.
Even if it is possible, the size of the thermoelectric conversion device is so large that the device is difficult to mass-produce at a low cost.
In the case of the manufacturing method disclosed in Japanese Patent Laid-Open Publication No. 22533/1998, a sufficiently stable yield cannot be expected with respect to plating transfer. Also, the device using laminated blocks having intermediate layers of an insulating material has a heat flow loss larger than that of the device having air between the elements, and cannot generate sufficient power when the temperature difference is small. Also, according to the method, the internal electrodes for series connection of the elements are joined directly to the elements and, therefore, the electrical resistance and the loss of heat are reduced. In practice, however, there is a need to polish the end surfaces of the blocks, so that it is difficult to manufacture the device at a low cost. Further, positioning marks cannot be used at the time of forming the internal electrodes, and it is difficult to form the internal electrode with sufficiently high reliability.
For these reasons, it is very difficult to manufacture a small, thin, compact thermoelectric conversion device with stability at a low cost by any of the conventional manufacturing methods.
It is an object of the present invention to solve the above-described problems, the present invention arranges a device as described below. That is, a thermoelectric conversion device of the present invention has p-type and n-type elements pn-junctioned by wiring of internal electrodes of two substrates opposed to each other, at least one of the p-type and n-type elements having, on only its one side, an electrode junction layer for joining to the internal electrode.
According to the present invention, the element is formed of a p-type or n-type thermoelectric material containing at least two elements selected from Bi, Te, and Sb deposited from an acid water solution by an electrochemical technique.
Further, the electrode junction layer is formed of a plurality of layers.
Also, the sectional shape of the p-type element or the n-type element is circular or elliptical.
Also, the surface of the respective element adjacent to the electrode junction layer has a surface roughness larger than the surface opposite from the surface adjacent to the electrode junction layer.
Also, the internal electrode wiring is formed of at least a conductive layer for internal pn junction, and a junction layer for joining the substrate and the conductive layer.
Further, the conductive layer is a metal layer having Ni as a main constituent.
Also, the substrates are Si substrates with a SiO2 layer.
As a manufacturing method for realizing the above-described thermoelectric conversion device, manufacturing methods described below are used. That is, a manufacturing method of the present invention comprises a first step of forming an internal electrode on a substrate, a second step of forming a mask pattern on the substrate such that at least a portion of the internal electrode is exposed, a third step of forming an element of a thermoelectric material on the exposed internal electrode by plating the substrate by an electrochemical technique, a fourth step of forming an electrode junction layer on the element, a fifth step of removing the mask pattern, and a sixth step of forming a pn junction by joining the electrode junction layer and an electrode provided on an opposed substrate opposed to the substrate.
Also, another manufacturing method comprises the steps of providing a metallic material on a substrate, forming a mask pattern on the substrate so that at least a portion of the metallic material is exposed, forming an element of a thermoelectric material on the exposed metallic material by plating the substrate by an electrochemical technique, forming an electrode junction layer on the element, removing the mask pattern, forming an electrode for pn junction by partially removing the metallic material, and joining the electrode junction layer and an electrode provided on an opposed substrate opposed to the substrate.
Still another manufacturing method comprises the steps of forming a first metal layer on a substrate, forming a plurality of internal electrodes by patterning on the first metal layer, forming a mask pattern on the substrate so that the internal electrodes are exposed at least partially, forming an element of a thermoelectric material on the exposed internal electrodes by plating the substrate by an electrochemical technique, forming an electrode junction layer on the element, removing the mask pattern, forming an electrode pattern from the first metal layer by etching the first metal layer using the internal electrodes as a mask, and forming pn junction by pressing and heating the electrode junction layer and an electrode provided on an opposed substrate opposed to the substrate so that the electrode junction layer and the electrode are joined to each other.
Further, a step of processing the p-type and n-type elements by a heat treatment is included.
As described above, since the pattern for forming the element is formed of a photosensitive resin, the element can be easily formed so as to be circular or elliptical. That is, stronger elements not easily broken by cracking, chipping or an external force can be realized.
P-type and n-type elements are directly shaped on a substrate by plating. Therefore, there is no need for an electrode junction layer between the elements shaped by plating and the substrate, and the electrode junction layer is provided on only one side of the element. The thermal resistance and the possibility of a contact failure at the time of joining for forming a thermoelectric conversion device, can be reduced. When the electrode junction layers on the elements are jointed to another substrate, since one side of the elements have already been fixed, there is no possibility of occurrence of a short circuit between the elements or a contact failure caused by a move of the elements. Thus, the reduction in yield and variations in qualities can be limited. Since the elements are caused to grow by plating, the junction interface between the elements and the electrode junction layers can be easily roughened suitably to have a surface roughness of several microns, corresponding to the crystal grain size of a Bixe2x80x94Te material. The area of contact with the junction layer is thereby increased in comparison with the flat junction surface, and the possibility of contact and bonding failures is reduced. As a result, the strength of junction between the electrode junction layers and the substrate is improved. Also, as a conductive layer for electrically connecting p-type and n-type elements in series, only a single metal layer is used. The process can therefore be simplified and a reduction in cost can be achieved. Also, two metal layers may be provided on one substrate while the single metal layer is used on the other substrate, thereby increasing the electrical conductivity as well as achieving a cost reduction effect. Thus, the invention is advantageous in an application to a thermoelectric conversion device using small elements. Also, since the plating substrate and the substrate used for the thermoelectric conversion device are the same, a further reduction in cost can be achieved.